As a method of producing phosphor (P)-doped single silicon crystals having high resistivity of greater than 10 .OMEGA..multidot.cm, there is a FZ-NTD method (floating zone - neutron transmutation doping). This method is carried out by irradiating neutrons generated from nuclear reactions to single silicon crystals which are grown through the FZ method (floating zone method), in order to transmute isotopes Si.sup.30 usually contained by about 3.10% in the single silicon crystals into Si.sup.31 and, then they are transmuted to P.sup.31, to thereby uniformly dope phosphor to the silicon single crystals. Since the single silicon crystals gown through the FZ method have an oxygen content (oxygen content according to the measuring standards specified by ASTM, 1981 edition: here and hereinafter) of below 5.times.10.sup.16 atoms/cm.sup.3, they can be used in the production steps of devices such as substrates for use in high voltage withstand power transistors, substrates for use in rectifiers, thyristors, etc., by applying heat treatment after the neutron irradiation, irrespective of the quality of neutron fluxes used for the irradiation, that is, the thermal neutron/fast neutron ratio.
Upon neutron irradiation, only the thermal neutrons contribute to the conversion of Si.sup.30 into P.sup.31. Fast neutrons having higher energy than thermal neutrons impinge against silicon atoms hat constitute a crystal thereby scattering the silicon atoms from the crystal position, stop after flying over a distance dependent on the initial energy and the property of the crystals to be irradiated, while losing the energy in the form of interstitial silicon atoms, whereby lattice defects corresponding to the amount of the fast neutrons are caused. If the single silicon crystals contain oxygen and if the interstitial silicon as the scattered interstitial atom and the interstitial oxygen are brought closer, binding is formed between them to possibly result in a lattice decect, that is, a A center defect. The A center defect is rapidly recovered by the heat treatment, by which the electric resistivity and the carrier life time are settled by the heating at 800.degree. C. to 1000.degree. C. in the case of single silicon crystals by the usual FZ-NTD method.
However, it is difficult and expensive, if possible, to obtain single silicon crystals of not less than 125 mm.phi. by the FZ method. As a method of overcoming this problem it has been tried to grow single silicon crystals of a relatively large diameter and low oxygen content by way of a T-MCZ (Czochralski method applying transversal magnetic field) and apply NTD (neutron transmuting doping) to the single silicon crystals obtained by the T-MCZ method.
In the case of applying the NTD process to the single silicon crystals grown by the T-MCZ method under certain conditions, as compared with the case of applying the NTD Process to the single silicon crystals grown by the FZ method, there is a first problem that although electric resistivity and the carrier life time are settled by applying heat treatment at 800.degree. C. to 1000.degree. C. to the single silicon crystals after the completion of NTD process, etched pits of not less than 10.sup.3 /cm.sup.3 are formed on the device in the subsequent device production step, a considerable leak current is present or current amplifying factor is reduced with respect to the NBS standard test device.
Further, since the single silicon crystals obtained by the T-MCZ method contain a greater amount of oxygen as compared with the single silicon crystals obtained by the FZ method although smaller as compared with the single silicon crystals obtained by he usual CZ method and, accordingly, A center defects are liable to be caused as compared with the FZ method. The A center defects are of course recovered by the heat treatment in the same manner as in the case of the single silicon crystals obtained by the FZ-NTD method.
In the NTD-applied single silicon crystals, since the electric resistivity and the carrier life time are recovered by the heat treatment under heating at a temperature from 800 to 1000.degree. C. or higher, it is supposed that the A center defects are also recovered. However, it is not actually confirmed whether the A center defects are actually recovered or not. Accordingly, there is a second problem that if characteristics of the single silicon crystals applied with the NTD process should vary, it is not possible to select those silicon wafers having favorable characteristics and not all of the silicon wafers have desired characteristics.
While on the other hand, as the application field where the silicon wafers for use in epitaxial growing are used is extended along with the enhancement for the performance of bipolar devices, MOS devices, as well a power devices, reduction in the crystal defects of silicon wafers has strongly been demanded. In view of the above, development in the production techniques for the silicon wafers has been progressed in which generation of crystal defects due to contaminations is coped with cleaning for the production circumstances, automation for the wafer handling device, use of highly pure chemicals etc., while crystal defects caused by fabrication defects generated in the wafer fabrication are coped with the improvement for the fabrication technics in addition to the abovementioned countermeasures. However, none of the countermeasures is quite sufficient and satisfactory.
In order to compensate the foregoing insufficient countermeasures, it is necessary to control the concentration of oxygen incorporated into a silicon wafer. For this purpose, the amount of oxygen intruding from a quartz crucible used in a single crystal pulling-up device into single crystals is severely controlled, and intrinsic gettering or wafer inside gettering (hereinafter simply referred to as IG), extrinsic gettering or wafer rearface gettering (hereinafter simply referred to as EG) or the combination of them is selected for use depending on the amount of oxygen generated and intruded into the silicon wafer in an additional heat treatment before the epitaxial growing step and heat treatment during the epitaxial growing step.
The term IG means herein the local fixation of impurities related to the amount of saturated oxygen in the silicon wafer determined by the heating temperature for the silicon wafer and the amount of deposited nuclei determined by the thermal hysteresis after the crystallization. Further, the term EG means the local fixation of impurities due to mechanical injuries, fine silicon polycrystals o fine Si.sub.3 N.sub.4 polycrystals applied under control to the surface of a silicon wafer on the side opposite to that applied with epitaxial growing (hereinafter referred to as a rearface).
The IG-gettering property is not favorable for silicon wafers with less oxygen deposition amount therein. However, if the amount o oxygen is increased excessively with an aim of enhancing the IG-gettering property, there is a problem that slips or the likes occur due to the deposition products upon heat treatment for the wafers. Then, since the application of IG is inadequate in the case of silicon wafers with the oxygen content of less than about 10.times.10.sup.17 atoms/cm.sup.3, EG is used in the case of such silicon wafers.
However, since a method of intruding fine powder of SiO.sub.2, SiC, Al.sub.2 O.sub.3, etc. into a high speed gas stream or high speed water stream and then blowing them to the rear face of a silicon wafer, or a method of rubbing the rear face of a silicon wafer with a brush comprising fine fibers made of organic material in admixture with the fine powder is used in the case of EG such as the mechanical rear face injuring method, for instance, there is a problem that it is extremely difficult to keep the cleanness at the rear face of the silicon wafer.
Accordingly, there is a third problem that application of EG as well as IG to silicon wafers with the oxygen content of lower than 10.times.10.sup.17 atoms/cm.sup.3 are inadequate.
Accordingly, a first object of this invention is to obtain conditions for the neutron irradiation dose capable of decreasing leak current upon doping P by way of neutron irradiation to single silicon crystals with low oxygen content manufactured by the CZ or MCZ method for overcoming the first problem described above.
A second object of this invention is to provide a silicon wafer with less relative leak current after heat treatment, and a device for selecting silicon wafers with less relative leak current after heat treatment, in order to overcome the foregoing second problem.
A third object of this invention is to provide a silicon wafer that can be applied with IG and a method of manufacturing a silicon wafer that can be applied with IG for overcoming the foregoing third problem.
The first object of this invention can be attained in a method of producing a P-doped silicon wafer comprising the steps of preparing a single silicon crystal by the MCZ method and having an oxygen content within a range from 5.times.10.sup.16 atoms/cm.sup.3 to 10.times.10.sup.17 atoms/cm.sup.3, and doping phosphor into said crystal by transmuting isotope Si.sup.30 contained in said crystal into P.sup.31, under a neutron irradiation to said crystal at, in an irradiation dose of fast neutrons less than 3.times.10.sup.16 /cm.sup.2.
The second object of this invention is attained in a silicon wafer mad of a P-doped single silicon crystals made by transmuting Si under neutron irradiation, in which said silicon wafer has a transmission intensity of not less than 30%, said transmission intensity being based on near infrared ray with a wavelength of 1.0 .mu.m to 1.4 .mu.m, as well as in a silicon wafer selecting device comprising a silicon wafer made of a P-doped single silicon crystals made by transmuting Si under neuron irradiation, an optical source for irradiating near infrared ray with a wavelength of 1.0 .mu.m to 1.4 .mu.m to said silicon wafer, and measuring means for measuring a transmission intensity of said near infrared ray transmitting said silicon wafer.
The third object of this invention is attained in another type of a silicon wafer for use in epitaxial growing, in which fast neutrons are irradiated at a dose of not less than 1.times.10.sup.12 /cm.sup.2, as well as another method of producing silicon wafer having a low density of A center type defects for use in epitaxial growing comprising the steps of preparing a silicon wafer having an oxygen content of not less than 5.times.10.sup.16 atoms/cm.sup.3, and irradiating fast neutrons to said silicon wafer at a dose of not less than 1.times.10.sup.12 /cm.sup.2 by a nuclear reactor having a thermal neutron/fast neutron ratio of not greater than 30.
The method of manufacturing a silicon wafer according to this invention can provide conditions for neutron irradiation dose capable of reducing the leak current upon doping P by way of neutron irradiations to single silicon crystals of a low oxygen content prepared by the CZ or MCZ method.
The silicon wafer according to this invention can provide a silicon wafer with less relative leak current after heat treatment and, in addition, the silicon wafer selection device according to this invention can select the silicon wafers with less relative leak current after heat treatment.
Another silicon wafer according to this invention can improve the IG-gettering property since the number of laminate defects induced by fabrication of silicon wafers can be decreased as compared with the case of not irradiating fast neutrons and, in addition, another method of manufacturing a silicon wafer according to this invention can provide another silicon wafer as described above.